Piezoelectric sound pressure microphone



June 7, 1949. F. MASSA 2,472,714

. PIEZOELECTRIC SOUND PRESSURE MICROPHONE Filed April 16, 1945 7 I 3 s 36 VII/ l. I V 7 51,

DEGREES PHASE Sill/'7' I F'RE 9. IN KC.

Jnventor Patented June 7, 1949 UNITED STATES PATENT OFFICE rmzoerec'rm'cSOUND PRESSURE MICROPHONE Frank Massa, Cleveland Heights, OhioApplication April 16, 1945, Serial No. 588,692

4 Claims." (Cl. 179-110) My invention is concerned with an improvedpoint before the microphone was introduced.

Microphones have been designed which achieve this overall requirementover portions of the audio-frequency range. The purpose of thisinvention is to describe a new type of construction which achieves thebasic requirements of a good 7 The convenience of having a really flatfre- :q'uency response characteristic is practically selfevident.Although variations from flatness of a microphone response are generallytolerated in the acoustic laboratory, .even though considerable time maybe employed in correcting for the microphone variationsover thefrequency :range, there are certain'measurements that can- :not :be madewhen such variations exist. Among such measurements are those involvingdistortion measurements on sound generators. The apparent harmoniccontent in the sound Wave would be very greatly dependent on the degreeof variation from flatness of the measurement microphone. Attempts tocompensate for irregularities :in a .microphone response by means ofelectrical networks, in addition to beinga generally cumbersome j ob,introduce electrical phase shifts in the system that, in themselves, maycause further errors when using themicrophone for the measurement .of"wave shape .of the sound wave. An important criterion, therefore, for.a good measurement standard is the extent of the frequency range overwhich its response is trul .flat.

2. Large dynamic range To .be generally useful, the electrical output ofa standard shouldbe linear with respect to the sound pres-sure over wideranges of amplitude. .A good standard should not berestrictedin itsfield of use only to soundpressure measurements has hitherto urement ofextremely high sound pressures such as would exist near the throat of ahorn loud speaker or inside enclosures such as sections of sound filters.(mufilers) or conduits. To realize a large dynamic range, it is ofprimary importance that the mechanical constants of the vibrating systembe inherently linear for large variations in applied pressure.

3. Small physical size An important criterion that must be met bya goodworking standard is that its physical size should besmallas comparedwith the wavelength of the sound pressure being measured. If thestructure is large enough to cause appreciable difiraction, themicrophone will show sensitivity variations with angle, and when placedin use it will require a knowledge of the direction along which .thesound wave is travelling as Well as accurate alignment of the microphonewith respect to the axis along which its calibration was made. Anotherdisadvantage of a large physical size is the possibility of standingwaves being established between the sound source and the microphone,which may make sound pressure measurements at high frequenciesimpossible.

For sound fields whichare not free progressing waves with a knowndirection of propagation, there is no assurance or being able to measuretrue sound pressure unless the microphone is physically small comparedwith'the wavelength. "For example, "if sound pressures are to bemeasured within an enclosure, such as the determination of pressuredistributionalong a conduit or in various compartments of a noisefilter, it is impossible to do it accurately with a microphone of suchphysical size that its active area integrates a number of differentpressure components acting over its entire face.

In some cases, even for work in the lower range of frequencies, it lsnecessary to have a microphone'which is physically very small, eventhough the wavelength is relatively large. A typical example is one inwhich the sound pressure measurements are to be made inside relativelysmall chambers in which the insertion of the micro phone must notappreciably upset the physical dimensions of the enclosure.

4. Smooth electrical impedance Even though a microphon may have a veryflat frequency response characteristic, it is important that itselectrical impedance characteristic be such that it easily lends itselfto use with conventional electronic circuits; otherwise it might becomeimpractical to make use of its flat open circuit sensitivity. Agenerally uncle sirable impedance characteristic is one in which largevariations in'magnitude occur over relatively small ranges in frequency.A very desir- 3 able type of electrical impedance for the microphone isone represented by a single circuit element over the entireaudio-frequency range so that it will be easily possible to make simpleadaptations of th microphone to input circuits for special uses. specialuses include the desirability of being able Some of the more common atrue plane surface to the sound wave by the to easily attenuate themicrophone sensitivityuniformly over the entire frequency range at theinput to the grid circuit to prevent overloading in cases whereextremely high sound pressure measurements are to be made. simplecircuit element will make it easy to vary the time constant of themicrophone circuit, if desired. Equally simple will be the possibilityof introducing filter sections at the input stage to permit specialmeasurements in limited frequency regions when improved signal to noiseratio may be necessary in measuring very weak sound fields.

5. High mechanical impedance One of the most important requirements fora sound pressure measurement standard is that its mechanical impedanceshall be extremely high over the entire audio-frequency range. Theimportance of this particular requirement is not generally appreciated.It is usually assumed that as long as a free field calibration of amicrophone is known, it may be used as a good working standard. Thefallacy in this reasoning is that the microphone only gives the correctinterpretation of sound pressure when used under the identicalenvironment in which the calibration was made. If the microphone is usedunder conditions other than free field (assuming free fieldcalibration), there is no assurance that the microphone will measuretrue sound pressure except in the frequency region in which itsmechanical impedance is high.

, If the mechanical impedance of the standard microphone is not higherthan the mechanical impedance of the environment in which it is used,the insertion of the microphone at the point where the sound pressuremeasurement is to be made may greatly disturb the sound pressure whichexisted at the point before the microphone was installed. Under suchconditions, the measurement obtained might be in very considerableerror.

One of the best means of achieving the important requirement of highmechanical impedance is to employ a stiffness-controlled mechanicalsystem. Of the numerous attempts which have been made to produce a goodstiffnesscontrolled working standard, the miniature condenser microphonedescribed by W. M. Hall in the Journal of the Acoustical Society ofAmerica, vol. i, page 83 (1932), and also by Harrison and Flanders inthe Journal of the Acoustical Society of America (Supplement to July,1932) has been generally accepted as best meeting the basicrequirements. My invention overcomes the limitations which are stillpresent in the miniature condenser microphone and thereby approaches theidealized requirements for a working standard to a much greater degree,as I shall show.

Other desirable features of my invention will become clear after readingthe objects and specifications which follow:

An object of my invention is to provide a microphone which measures truesound pressure in a sound field Without disturbing the pressure wavephone was introduced.

Also, a

- pressure sensitive portion of the microphone, thus completelyeliminating any cavity ahead of this surface such as is caused byclamping rings in conventional types of microphone structures.

A further object of my invention is to produce a mechanical vibratingsystem in a sound pressure microphone which is true stiffness controlledover the entire audio-frequency range extending well beyond 15,000cycles.

Another object of my invention is to provide a microphone which behavesas a simple vibrating mechanical system in which the fundamentalmechanical resonance occurs very much higher than 20,000 cycles.

Still another object of my invention is to provide a microphone in whichthe electrical impedance of the unit is practically that of a purecondenser over the entire audio-frequency range to at least 20,000cycles.

A further object of my invention is to provide a microphone whosegenerated voltage is proportional to sound pressures over very largeranges of sound pressures exceeding millions of dynes per squarecentimeter.

Another object of my invention is to produce a structure for use as asound pressure measurement standard in which the stability of the systemis very high and not subject to change with time and room temperaturevariations.

Still another object of my invention is to produce a true pressuremicrophone standard which is very easy to use and convenient to operate.

Another object of my invention is to provide a microphone which is verysimple to use as a measurement standard and not subject to the necessityof requiring polarizing voltages for its operation.

A further object of my invention is to provide a method of assembly ofthe component parts of the Still another object of my invention is topro- 1 vide a preamplifier of such design that the microphone may beplaced in a sound field with minimum obstruction offered to the sound.

Another object of my invention is to provide a novel means ofshock-mounting the microphone unit from the amplifier housing.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The invention,itself, however, both as to its organization and method of operation, aswell as advantages thereof, will best be understood from the followindescription of several embodiments thereof, when read in connection withthe accompanying drawings, in which- Fig. 1 is a partially cut-away viewlooking end-on at the microphone.

Fig. 2 is a longitudinal section through the microphone taken along 2-2of Fig. 1.

Fig. 3 is an enlarged view showing the details of the terminalconnections of the microphone.

Fig. 4 is an endview showing the assembly of h. group of piezo-electiiccrystal plates as employed in the microphone construction.

Fig. isa side view of Fig.4.

Fig. 6 is a .view showing a single crystal plate of Fig. 5 with aconducting surface or electrode applied to a portion of the crystalface.

Fig. 7 shows the crystal plate of Fig. '6 after electrical contactstrips are attached.

Fig. 8 shows an importantstep intheassemb1y procedure for the microphonein which very accurate alignment of the crystal assembly to the baseplate is automatically maintained.

Fig. '9 is an end view of a portion of the device shown in Fig. 8.

Fig. is a graph which gives the data showing the deviation from truestiffness control at the higher audio'frequencies of :a small stretched-diaphragm as comparedwith no deviation for'the microphone describediinthis specification.

Referring more particularly to the figures, Rig. .1 .showsan end view of"the microphone assembly, and Fig. 2 is a longitudinal section throughthe assembly. The basic components which make up the microphone are athin diaphragm i, an assembly of piezo-el'ectri'c crystal plates 2, arigid :base 3, and an outer housing '4. The crystal leads 5 pass througha pair of electrically insulating bushings '6, which are forced fit intothe base 3 A pair of conducting terminal pins Tare driven mto thebushings B establishing permanent connection to the crystal leads 5. Theouter housing 4 is a cylindrical shell that is :made a forced fit overthe undercut shoulder 3 of the 13886 3. corners is cut through theopposite end of the housing 4, as indicated by the cut-away portion ofl, to result in a small clearance between the crystal assembly 2 and thesides of the opening.

One of'the basic features of the construction of the microphone is theobta'inment :of a high "degree of mechanical precision in the assemblyso that the final performance will be stable over a :long period Oftime. To realize this precision, the constructionis builtup -around thebase piece Q whose surface :to which the crystal assembly .2 is cementedis made perfectly flat. The shoulder 3 on base 3 is machined exactly atright angles to the flat surface on which the crystal assembly iscemented. Starting with these two accurate surfaces on the base3, theentire microphone as- :sembly is built up, as will be described later inconnection with Fig. 8 and Fig. 9.

To bring out the electrical terminals 1 for con- .nection to the crystalleads, a pair of insulating bushings 6 are forced into the base 3, asshown in the enlargeddetail in Fig. 3. Each of the flex- ..ible leads 5passes through a hole in one of the insulating bushings 6 andaconducting terminal .pin 1 is driven into the hole in each of the bush-:in-gs 6, establishing permanent contact with the lead 5 andsimultaneously providing a. pair of external contact tips through whichexternal connection may be established to the microphone.

Fig. 4 and Fig. 5 are top and side views of the crystal assembly 2 whichis the active element of the microphone. I prefer to employ an evennumber of crystal plates in the crystal assembly so that the two outsideplates will then be at common potential and can be kept nearest toground potential when in use, if desired, thus reducing the magnitude ofthe leakage capacity -:between the crystalassembly and ground, which isadvantageous in minimizing sensitivity loss in A rectangular openingwith chamfered of crystal material will be discussed later.

'the microphone'circuit. Six'plates 8, 9, i0, H,

12, and iii are indicated in the particular crystal assembly shown inFigs. 4 and 5. Although I prefer to use several slabs of crystalconnected in parallel in order to realize a suiilciently high value ofcapacity for the assembly, it is also possible to employ :a singleithi'ck crystal plate for the active element. If this is done,thecapacity of the element will be very low and the leakage capacity fromone side of the crystal to the housing will introduce a shuntingsensitivity loss across the crystal. The low value of capacity will alsointroduce otheradifil'culties in the ampliyfier circuit to permit hatresponse to very low frequencies.

Common potential faces of the crystal are marked in Fig. .4. Thesix-"common sides are-brought out to the right, as shown, by threecommon'leads 15. The topand bottom leads l5 pass over insulating stripsl5 'whichare attached to the crystals, asshown in Fig. 5, and join withthecommonrcenter lead I5to connect .to the flexible conductor 5.Asimilar construction is followed for the leads :of opposite polaritywhich arebrought-out to the left of Fig. 4. Four crystal leads 15 arenecessary, and three insul'ating pieces I6 are used for the .lefthandconnection, as is obvious from :the details of Fig. .4. The two endcrystalsfi and 13 are chamfered, as shown, to avoid the necessity .ofcutting right angle corners in the housingrl of- Fig. 1, which, if done,would make the .radial distance of the four corner regions over whichthe diaphragm is ce-- mented too "small unless the outside diameterincreased. 7

The construction of the individual crystal plates is'shown for a typicalplate 10 in Fig. 6

and Fig. I. The piezo electric crystal plate 10 is an expanderplate inwhich a pressure applied along one axis causes a charge to be developedalong aright angle axis. .-;Typical types of crystal plates that may beemployed are 45 X-cut or 45 Y-cut Rochelle salt, 45 Z-cut primary ammo-.niumphosphate, 01"71" ZX-cut quartz. The choice In preparing thecrystal assembly 2 the plates, for

exampleplate M3,:are cut of such dimensions to produce the desiredsensitivity, impedance and resonant frequency for the microphone. On apair of opposite faces, a-conducting film M is applied, preferably withmargins near the top and bottom edge, as shown, so that the strayleakage capacity produced in the microphone between the crystal assembly2, the base 3.rand the diaphragm -l is reduced toa minimum. Theconducting film -l=4 may be applied by the method described in myco-pending application, Ser. No. 522,196, filed February V10, 1944, nowabandoned, or by the evaporation of gold or other metals in a vacuum, orby cementing thin conducting foil to the bare crystal, or by painting aconducti'ng cement over the desired surface. In order to bring outsuitable electrical connection to the crystal electrodes, the thincontact strip 15 is attached .over an edge of the electrode 14, as shownin Fig. 7. .A strip of similar thickness 1-511. is placed along theopposite edge of the electroded surface, as shown in Fig. 7, so thatwhen the plates are assembled as agroup, as shown in Figs. 4 and 5, eachplate will remain parallel to each of the other plates.

When the crystal :plates are assembled as a group, as shown in Figsmiand 5, the plates are cemented and held in a .fixture which keeps allsurfaces of the assembly mutually perpendicular. .A cementaloaded-with-a metallic powder to in- 7 crease the conductivity ispreferably employed when assembling the crystal plates so that thestrips I 5, I which make common connection to its neighboring electrodesurface will make the connection with negligible contact resistance.

After the crystal assembly 2 is complete, the surface on the end whichis to be cemented to the base 3 is made perfectly fiat by dressing itoff while the assembly is rigidly held in a fixture which holds the longsides of the crystals exactly at right angles to the end surface beingdressed. The next stage in the construction consists in cementing theprepared end of the crystal assembly 2 to the flat surface of the base3. To maintain extreme accuracy in performing this operation, thefixture shown in Fig. 8 is employed. After the crystal assembly 2 isapproximately located on the base 3, with cement positioned between thecrystal assembly and the base, the guide I! is carefully placed over thecrystal and the crystal adjusted until the guide l1 engages the shoulder3 of the base 3. Locating means, not shown, are provided so that apredetermined location is maintained for the position of the guide pieceI"! around the periphery of the base 3. A top view of the guide I! isshown in Fig. '9 indicating the four fiat guide surfaces H which clearthe crystal assembly by a small margin, thus causing very accuratelocation of the crystal with respect to the base 3.

With the guide I! in place to maintain accurate alignment of the crystalassembly 2 with respect to the base 3, the entire sub-assembly is placedin a small rigid press consisting of a framework 2| and a screw 20 whichapplies pressure through the plate I9 and the pressure pad l8 to causeintimate contact of the crystal surface to the base 3. While theassembly is under pressure, the cement is allowed to set to produce anintimate bond between the entire area of the end of the crystal assembly2 and the base 3. After the assembly of the crystal and base iscompleted, it is possible to visually inspect the joint in order toobserve that the work has been properly carried on. The importance ofhaving a good intimate and continuous joint between the crystal and thebase is due to the fact that any imperfection in this joint is reflectedthrough as a change in the mechanical impedance of the system withresultant variations in the performance of the microphone.

Having accurately made the assembly of the crystal to the base, thehousing 4 is pressed into position around the crystal assembly, asindicated in Fig. 2. Due to the accurate alignment of the crystalassembly on the base, the rectangular opening on the diaphragm end ofthe housing will allow a uniform small clearance, preferably less than"1 5", to remain between the crystal assembly and the edges of theopening, as shown in the cut-away portion of Fig. 1. The reason forhaving such a small clearance is that when the diaphragm l is assembledin place there will be no appreciable areas of diaphragm which areunsupported and which might give rise to spurious variations in theresponse at the higher frequencies.

In order to insure a perfect assembly of the diaphragm l to the surfacesof the crystal assembly 2 and the flat portion of the end of the housing4, I find it advantageous to make the crystal assembly 2 slightly longerthan necessary and to dress off the excessive crystal material byrubbing the end of the assembly on a fiat abrasive surface until theexposed crystal and housing surfaces are brought into the same plane. Inthe case of quartz which is more difficult to wear down, it is possibleto attach securely a thin sheet of Bakelite or similar substance on oneor both ends of the crystal assembly and during the final dressingoperation the exposed Bakelite surface may be more easily brought intoaccurate alignment with the plane of the exposed flange surface of theouter housing to prepare the unit for the assembly of the diaphragm.

The diaphragm l is relatively thin and does not contribute to themechanical constants of the vibrating system. It merely acts as anenclosing sound sensitive surface for the microphone. After bringing theexposed crystal surface and the housing flange into the same plane, thesurfaces are coated with cement and the diaphragm l, which is likewisecoated with cement is put in place, as shown in Fig. 2. The completeassembly is then mounted in the same type of press 2| as shown in Fig. 8and the unit is allowed to set under pressure, thus completing theassembly. In the choice of material for the housing 4, I prefer a metalhaving a coefficient of expansion nearly equal to that along the lengthof the crystals so that there will be a negligible static stress set upalong the axes of the crystals due to changes in temperature. For amicrophone employing primary ammonium phosphate as the crystal element,a housing of cadmium would be the most desirable since the coefiicientof thermal expansion for cadmium is practically identical with thecoefficient of expansion of the crystal along its length. Zinc wouldalso be usable with a coefficient of expansion approximately 10% lessthan the coefficient for the crystal. For the case of quartz in whichthe coefficient of thermal expansion is very low, one of the nickelalloys having low temperature coefficients could be selected for thehousing material.

It is to be noted that the method of attaching the diaphragm does notrequire the use of a clamping ring; therefore, this constructioncompletely eliminates any cavity ahead of the diaphragm such as occursin conventional diaphragm assemblies. The absence of such a cavityprevents the occurrence of any acoustic resonance that would otherwisebe present in the response in the higher frequency region.

A microphone built as shown in Fig. 2, in which the outside dimensionsof the housing are approximately diameter by la? long, offers a fairlygood compromise between smallness and sensitivity. For such a structurea clearance of the order of between the crystals and the housing openingwill give satisfactory performance. It is no trouble to halve thedimensions of the structure if a still smaller physical size is desiredin cases where most of the work is to be carried on above 15,000 cycles.If this is done, however, both the capacity and the sensitivity willdecrease, which means that it will become more diflicult to work withthe amplifier circuits at the extremely low audio frequencies. This, onthe other hand, would be of no serious concern if the unit were to beused primarily at the higher audio frequencies.

Although several types of crystals may be used in the microphone, Iprefer to employ primary ammonium phosphate as a satisfactory substancefor most general purpose applications. X-cut Rochelle salt would havehigher sensitivity but its electrical impedance and mechanical stiffnesswould vary with temperature which would offer some trouble in caseswhere filter circuits would mins be desired at'theinput tothefirstAnother disadvantage. of Rochelle. salt-is. its 'rela. tively lowmelting point: of approximately 135? El, which would limit the fieldofapplicationof the "standard. Quartz. would have a muchhigher. meltingpoint than the primary ammonium phos phate; however, a microphoneemploying quarts of the same physical dimensions and designed for thesame capacity willl havc'approximatcly 15 db. less sensitivity thanprimary ammonium phosphate.

' The melting point of primary ammonium phosphate is'above 350 F. and amicrophone emplaying this crystal. can be used to at least 200- F".without difficulty. Since this particular crystal material shows adefinite volume conductivity which is a: iunctlon of the impurities inthe subs stance, it is lmportant to choos'e'the material for highresistivity if fiat response is desired down to cycles. Using primaryammcnium Phosphate in the microphone above described, in. which theoverall dimensionsarc diameter by H long, gives an electrical capacityequal to approximately 100 mi. and av sensitivity of about 26microvoltsfdyne/cm. pressure applied to the diaphragm throughouttheentire audio-frequency range.

The use of the piezo-electric construction just described has severaladvantages over the small stretched diaphragm condenser microphone foruse as a sound pressure measurement. standard. Due to the limitation onthe. strength of materials, it is not possible to stretch a practicalsize diaphragm to resonate above 20 kc. Even at 15 km, a steel diaphragm/2" diameter requires a peripheral tension. of about 70,000 lbs/sq.111., while for an aluminum or an. aluminum alloy diaphragm, a tensionof. about. 25,0001 lbsJsq. in. is necessary. These stresses are: notonly dangerously close to. they yield point of the materials, but

the possibility of setting lip-such a high tension and keeping it fromchanging over long. periods of time and over wide ranges of. roomtemperature variations is extremely difficult.

As a result of the relatively small mass of a stretched diaphragm. ascompared with'thecffective mass of the vibrating crystal in themicrophone herein described, the small condenser microphone deviates toa larger degree from true stiffness control below its resonant frequencythan would be the case if its vibrating mass were larger. An indicationof the amount of the deviation from a true stiffness may be learned fromthe change of phase angle of the mechanical impedance of the system withfrequency. For a simple vibrating mechanical system, it is well knownthat the phase angle is given by where:

A true stiffness-controlled vibrating system would show a value of sfrom Equation 1 equal to --90 over the entire frequency range ofoperation. In order to compare the microphone described in thisspecification with a typical miniatore stretched-diaphragm type. the.amount. of phase shift. from; a pure stifiness reactance has beencomputed fromEquation 1 assuming that the stretched diaphragm has anelf'ective area of 1 sq. cm., an eflective mass of .001 mm, and aresonant frequency of 12 kc. The result is shown by the dotted line inFig. 1d. The heavy solid line at the degree phase shift line in Fig. 10showsthe computed phase'shift for the con: stants of the, piezo olectricmicrophone herein described. From. Fig. 10 it. is evident that thestretched diaphragm type deviates from the ideal stiffness-controlledsystem at frequencies well within the v audiofrcquenoy range, whereasthe described microphone shows no deviation at all throughout the entireaudio range: to '20 kc. The computation of the dotted curve has.neglected themechanical damping. resistance in the system which, iftaken into account, would show a greater phase, shift; error thanindicated. If the stretched diaphragm is larger in diameter or resonatesbelow 12 be, which is more typically the case with practical structures,the. phase, shifterror will be alsosmuch worse than indicated by thedotted curve-in Fig. 10.

The maximum s und. pressure which can be measured by the. stretched.diaphragm type; of microphone is. limited by the relatively small per.-missible deflection-or the. diaphragm before non! linearity setsln.These non-linearities occur for soundpressures of the order of athousanddynes/cm. where the alternating amplitudes. will already amount to a fewper cent of the. fixed diaphragIn:v spacing... In. the described microiphone employing either primary ammonium p osphate crystals or quartz, helineari y: s limited only by the linearity or the complianceot thecrystal substance; and since these materials obey Hookes law topressures of the order ofth usands of pounds: per square inch, there isno practical limitation: to the use of themicrophcne up to soundpressures of many milliondynes/ cmF. Because the resonant frequency ofthe micro. phone assembly has been placed very much. higher than 20 kc.by employing a crystal length less than one inch. the electricalimpedance of the microphone is essentially that of a simple condenserover the entire audio-frequency range beyond 20 kc. The advantage ofhaving such a simple circuit element for the microphone impedancepermits great flexibility in the use of the microphone in combinationwith special input circuits.

Since the voltage generated in the crystal element is proportional tothe pressure acting on the diaphragm over the entire audio-frequencyrange to beyond 20 kc. and also because of the very accurate assemblyprocedure which has been described, the resultant microphone output isabsolutely uniform with respect to pressure acting on the diaphragm overthe entire frequency range to 20 kc. For the diameter unit, somedifiraction occurs at the higher audio frequencies which may be of noserious concern for the large majority of uses. An-

other microphone built along the same lines disclosed, but having anoutside diameter of will practically eliminate most of the diffractionproblems throughout the frequency range to the neighborhood of 20 kc.When such an extremely small physical structure is necessary for veryspecial high-frequency sound pressure measurements, the resultant lossin sensitivity will have to be tolerated.

In applications requiring pressure measure- "11 ments at relatively hightemperatures, the use of quartz as the active crystal assembly isdesirable, again producing a somewhat lower sensitivity, as previouslydescribed.

Although I have chosen certain specific procedures for producing amicrophone possessing new advantages as a sound pressure measurementstandard to illustrate the basic features of my invention, it will beobvious to those skilled in the art that numerous departures may be madein the specific details for executing the required functions, and I,therefore, desire that my invention shall not be limited except insofaras is made necessary by the prior art and by the spirit of the appendedclaims.

I claim as my invention:

1. In the method of making a microphone comprised of expander crystalmeans which comprises the steps of establishing at one end of saidcrystal means a plane perpendicular to a direction of expansion of saidcrystal means, mounting said crystal means with its said plane faceagainst a plane face of a base, connecting a housing to said base withsaid housing around said crystal means, the unattached end of said rhousing lying approximately in the plane of the unattached end of saidcrystal means, facing-01f the unattached end of said housing and crystalmeans until they are brought into a common plane, and attaching a planardiaphragm to said crystal means and said housing at said end which hasbeen faced-off.

2. In the process of manufacturing a transducer, the steps of connectingpiezo-electric crystal means to a base, afilxing to the free end of said7.

crystal means a layer of material which has a higher mechanicalimpedance than the crystalline material, connecting an enclosing housingto said base, the free end of said housing defining a planeapproximately parallel to the surface of said afiixed material, facingoif said affixed material until it lies in the same plane with the endof said housing, and connecting a diaphragm to the free end of saidafiixed material and to said housing to seal said housing.

3. In a microphone, a base member having a 1 2 planar face, a pluralityof expander plates of piezo-electric material connected together andmounted on said base with an axis of expansion and contraction normal tothe plane of said face, a housing extending around said expander platesand mounted on said base, the end of the housing away from said baselying in a plane parallel to the plane defined by the end of theplurality of expander plates, a layer of material attached to the end ofthe plurality of expander plates, the

mechanical impedance of said layer being greater than the mechanicalimpedance of said plurality of expander plates, the non-attached surfaceof said layer of material lying in a plane defined by the end of saidhousing, a diaphragm, and means connecting said diaphragm to said layerof material and to said end of the housing away from said base, wherebythe outside surface of said diaphragm presents an unbroken plane to theperipheral edge of said housing.

4. The invention set forth in claim 3 further characterized in thatinsulated-terminal means are provided in said base member forestablishing electrical connection from one side of said base member tosaid plurality of expander plates mounted on said base member.

FRANK MASSA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,813,461 Nicholson July 7, 19311,939,302 Heaney Dec. 12, 1933 2,096,826 Schrader Oct. 26, 19372,138,036. Kunze Nov. 29, 1938 2,181,132 Kallmeyer Nov. 28, 19392,388,242 Arndt Nov. 6, 1945 2,393,429 Swinehart Jan. 22, 1946 2,405,604Pope Aug. 13, 1946 FOREIGN PATENTS Number Country Date 477,740 GermanyJune 20, 1929

